It is known in the art of optical compensation that the phase retardation of light varies according to wavelength, causing color shift and contrast ratio reduction. This wavelength dependence (or dispersion) characteristic of the compensation film may be taken into account when designing an optical device so that color shift is reduced and contrast ratio increased. Wavelength dispersion curves are defined as “normal (or proper)” or “reversed” with respect to the compensation film having positive and negative retardance (or retardation). A compensation film with positive retardance (positive A- or C-plate) may have a normal dispersion curve in which the value of phase retardation is increasingly positive toward shorter wavelengths or a reversed dispersion curve in which the value of phase retardation is decreasingly positive toward shorter wavelengths. A compensation film with negative retardance (negative A- or C-plate) may have a normal dispersion curve in which the value of phase retardation is increasingly negative toward shorter wavelengths or a reversed dispersion curve in which the value of phase retardation is decreasingly negative toward shorter wavelengths. Exemplary shapes of these curves are depicted in FIG. 1.
Wave plates are customarily named as follows in accordance with their refractive index profiles: positive A-plate: nx>ny=nz; negative A-plate: nx<ny=nz; positive C-plate: nx=ny<nz; negative C-plate: nx=ny>nz, wherein, nx and ny represent in-plane refractive indices, and nz is the thickness refractive index.
The above wave plates are uniaxial birefringent plates. A wave plate can also be biaxial birefringent, where nx, ny, and nz all have different values; it is customarily referred to as a biaxial film.
An A-plate is a wave plate commonly used as a retarder in an optical device. It is a birefringent material capable of manipulating the polarization state or phase of the light beam traveling through the medium. The A-plate optical retarder has a refractive index profile of nx>ny=nz, wherein nx and ny represent in-plane refractive indices and nz represents the thickness-direction refractive index. Such a wave plate exhibits a positive in-plane retardation (Re) as expressed by Re=(nx−ny)×d, wherein d is the thickness of the wave plate. Re is also often denoted as Ro.
An A-plate having in-plane retardation (Re) equal to a quarter of a light wavelength (λ), Re=λ/4, is called quarter wave plate (QWP). A quarter wave plate is capable of converting an incident linearly polarized light into circularly polarized light. Thus, a quarter wave plate is commonly used in combination with a linear polarizer to provide a circular polarizer in an optical device. Circularly polarized light has been used in polarized three-dimensional (3D) display systems to produce stereoscopic image projection. Circular polarization has an advantage over linear polarization in that viewers are able to tilt their heads and move around naturally without seeing distorted 3D images. Such 3D display systems require viewers to wear glasses, commonly referred to as 3D glasses, equipped with circular polarizing films in order to see 3D images. Recently, there has been much increased interest in 3D consumer products such as TVs and computer displays. Thus, there is a demand for improved 3D glasses with circular polarizing films. Specifically, there is a need for a quarter wave plate having normal wavelength dispersion, which has been found to have the utility for 3D glasses to improve the viewing quality. It is known that such quarter wave plates can be achieved by using polycarbonate or cyclic polyolefin. However, in a device based on such quarter wave plates, a cellulose ester film is required to protect the polyvinyl alcohol based polarizer. It would be advantageous if the quarter wave plate is based on cellulose ester film and can also function as a protective film for the polarizer. Accordingly, this invention is further directed to quarter wave plates based on cellulose ester.
In order to have a normal wavelength dispersion curve, the in-plane retardation (Re) of a quarter wave plate should satisfy the following equations:Re(450)/Re(550)>1 and Re(650)/Re(550)<1wherein Re(450), Re(550), and Re(650) are in-plane retardations at the light wavelengths of 450 nm, 550 nm, and 650 nm respectively.
The positive A-plate, however, also exhibits a negative out-of-plane retardation Rth, which is defined as Rth=[nz−(nx+ny)/2]×d with a value of |Re/2| arising from its orientation. The term “|Re/2|” means the absolute value of Re/2. This characteristic can be beneficial when a negative Rth is desirable in an optical device. For example, in a vertically aligned (VA) mode liquid crystal display (LCD), the liquid crystal molecules in the LC cell are aligned in a homeotropic manner, which results in positive out-of-plane retardation. A wave plate with a negative Rth, thus, can provide an out-of-plane compensation in addition to in-plane compensation in VA-LCD. In other types of devices, such as in-plane switch (IPS) mode LCD and 3D glasses, however, the Rth exhibited in the A-plate is not desirable since it can give rise to phase shift in off-axis light and lead to light leakage. Thus, there exists a further need in the art to provide a quarter wave plate having reduced out-of-plane retardation for improved viewing quality.